Method of insulating electrical conductors



Dec. 3, E966 R G. FLOWERS ET AL 3,291,639

METHOD 0F INSULATING ELECTRICAL CoNDUcToRs 3 Sheets-Sheet 1 Filed Feb. 25, 1963 fic Wr my w, n www5 W L J/ @www /0 f/@Mf f 2 0 j -m 20C /W/f E, -wm 2./ p m -www M o Nolo. -m AU 6 01 Dec. 13, i966 R. G. FLOWERS ETA'- 3,29l639 METHOD OF INSULATING ELECTRICAL CONDUCTORS 3 Sheets-Sheet 2 Filed Feb. 25. 1965 I l l 210 220 230 V1 M v l n0 M w w a ,w m. 5 W k H l K n m w: m

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Dec. 3, 1966 R G, FLOWERS ET AL 3,291,639

METHOD 0F INSULATING ELECTRICAL coNDUcToRs Filed Feb. 2 5. 1963 w m w w S m 14o 17o 18ov rfMPm/n//ara amas S'erer,

United States Patent O 3,291,639 METHOD F INSULATING ELECTRICAL CONDUCTORS Ralph G. Flowers, Pittsfield, and Thomas L. Sherer, Richmond, Mass., assignors to General Electric Company, a

corporation of New York Filed Feb. 25, 1963, Ser. No. 260,641 4 Claims. (Cl. 117-232) This invention relates to insulating electrical conductors and more particularly to a novel process of insulating electrical conductors and the conductors insulated by such process.

It is well know to those skilled in the electrical insulating art to apply a type of enamel film insulation to electrical conductors, such as Wires and the like. In general, the electrical conductor is coated in a wire mill in which an elongated wire is taken from a supply reel and passed continuously through a bath of insulating coating liquid. The wire with the coating material adhering thereto is then passed through a drying or curing oven where the insulating coating is hardened or cured. The wire is then passed through the insulating bath and the drying oven in repeated stages until a coating of the desired thickness has been built up on the electrical conductor. The coated insulated wire is then usually wound on a take-up reel for storage purposes until needed.

In coating electrical conductors in this manner, the curing of the insulating enamel material results in a chemical reaction in which one of the constituents of the enamel reacts with another constituent to form a cross linked, tough, flexible film insulation. In this process water and/or other volatile by-products are evolved. These by-products must be substantially eliminated in order to provide a good electrical insulation. It is the usual practice to try to eliminate these volatile byproducts by use of the drying oven which provides a high temperature cure to the various layers of film insulation.

In order to substantially eliminate these volatile byproducts during the curing of the film insulation, and to produce a good electrical insulation, excessive temperatures are required which have an undesirable side effect. This undesirable side effect is that these high curing te-rnperatures have an `adverse effect on the physical properties of the film insulation. In general, it is found that the film insulation tends to become embrittled when subjected to the high curing temperatures in the normal curing oven, thereby rendering the insulated electrical conductor subject to damage from winding or abrasive action. Thus in the present-day insulating art, the :temperature of the drying oven and the time of passage of the coated electrical conductor therethrough is set as a compromise between the substantial elimination of the volatile by-products to provide optimum electrical properties, and the physical degradation of the insulating film which can be accepted for insulated electrical conductors under the particular circumstances of intended use for the particular conductor.

It has been recently discovered that the embrittlement of the film insulation is in part caused by the combination of the oxygen with the curing film at the curing temperature. It has further been discovered that if the detrimental effects of the oxygen can be eliminated or reduced, that a more Ioptimum chemical cure to obtain desired electrical properties can be obtained without any adverse effects on the physical properties of the film. A further discovery has been made in that insulated film on wire that has previously been cured in the normal manner may have its electrical properties substantially improved by a further curing in which the detrimental effects of oxygen on curing are eliminated.

Patented Dec. 13, 1966 ICC It is, therefore, one object of this invention to provide an improved method of insulating electrical conductors.

It is a further object of this invention to provide an improved film insulated electrical conductor.

A further object of this invention is to provide a new curing method for film insulation on electrical conductors.

A further object of this invention is to provide an additional curing method for previously cured yfilm-insulated electrical conductors.

A further object of this invention is to provide an improved film insulation on electrical conductors by subjecting such conductors to a second improved curing.

In carrying out this invention in one form thereof, an improved film-insulated electrical conductor is provided in which the film insulation is cured in a nonoxidizing medium. In another form of the invention a cured filminsulated electrical conductor is subjected t-o a further cure in a nonoxidizing medium.

The invention which is desired to be protected will be clearly pointed out and distinctly claimed in the claims appended hereto. However, it is believed that this invention and the manner of carrying it out, as well as the -manner in which its various objects and advantages are obtained, together with other objects and advantages thereof will be more clearly understood by reference to the following detailed description of various preferred embodiments thereof, particularly when taken in connection with the accompanying drawings, in which:

FIGURE l is an elevation view partly in section showing one form of this invention applied to a continuous coating apparatus;

FIGURE 2 is an elevation view partly in section showing one form of apparatus according to this invention for providing a second cure to previously cured film-insulated electrical conductors;

FIGURE 2a is a partial sectional View showing a modification which may be used with the apparatus of either FIGURE 1 or FIGURE 2;

FIGURE 3 is a group of curves showing the dissipation factor of cured polyvinyl formal coated electrical conductors at various temperatures, and illustrating the improvement which may be obtained by means of this invention;

FIGURE 4 is another group of curves showing the dissipation factor of cured polyvinyl formal coated electrical conductors at various temperatures pointing out the improvements which may be obtained by one form of this invention;

FIGURE 5 is a group of curves of an epoxy enamel film insulation cured on an electrical conductor showing the improvements which may be obtained by means of one form of this invention;

FIGURE 6 is a group of curves of a polyester type enamel lm insulation cured on an electrical conductor to show the improvements which may be obtained by means of one form of this invention; and

FIGURE 7 is a group of curves showing the improved aging characteristics of a polyvinyl formal enamel coated insulated electrical conductor when cured according to one form of this invention.

As earlier noted, it has -been discovered that the electrical properties of film insulation on electrical conductors can be substantially improved without detrimental effects to their physical properties when the film-coated wire is cured in a nonoxidizing `atmosphere or medium. FIGURE l shows one form of coating apparatus which may be used to carry out the method of this invention. A wire coating mill is shown in FIG. 1 for coating continuously moving electrical conductors. As shown in lFIG. l, the wire mill comprises Ia base member 10 having a quantity of a suitable insulating coating'liquid 12 therein,- and a baking or drying oven 14 mounted on top of the base member 10. An electrical conductor 16, to be coated in the wire mill, is wound upon a supply reel 18 and is led from such supply reel through a guide sheave 20 mounted in the base member 10 within the liquid insulat-ing coating 12, in the manner shown. Wire 16 is led around the guide sheave 20 where it is provided with the coating material which adheres thereto and then passes through a die device 22, such as is Well known to those skilled in the art, and extends up through the drying oven 14 where it passes around a second guide sheave 24. The wire 16 is then returned to the base member back through the insulating liquid 1-2 about a second guide sheave, behind guide sheave 20, where a second coating of the insulating liquid is applied thereto, again, through the drying oven and around a further guide sheave 24 at the top. The wire 16 passes through the coating and drying apparatus in repeated stages until a coating of the desired thickness is built up on Wire 16. The'coated wire is finally conducted to a take-up reel 26.

The novel feature of the wire mill disclosed in FIG. 1 is in the drying oven 14. As shown, the drying oven 14 comprises refractory side Walls 28 and 3G. Each of the side Walls is provided with electrical heating means 32, as is well understood by those skilled in this art. Similar end walls (not shown) are also provided. The drying ovenv is securely 'attached to the base member 10 of the wiring mill. As is shown in the drawing, a substantially air-tight sealing member 34 is provided over the open portion of the base member 10 contacting both the base member and side 30 of the drying oven. Sealing member 34 is provided with openings 36 and 38 therein to allow the wire 16, as it passes from the take-up reel y18l and from the guide sheave 24, to enter the base member 10. As will be understood, each of openings 36 and 38 are provided with sealing means, such as rubber gaskets or the like 37, 39, so yas to prevent entry of air into the drying oven 14. The top of the drying oven is similarly provided with a cap member 40. The cap member 40 is also provided with openings 42 therein or the passage of wire 16 therethrough. Each of openings 42 is similarly provided with sealing means-in the form of rubber gaskets or the like, 43, to prevent the entry of air therein. Member 34 is provided with an opening therein, indicated at 44, and a neck member 46 extends from the opening. The neck member 46 leads from the sealing member 34 and the drying oven 14 to a device 48 Which may be either a vacuum pump or which may be a device for providing a nonoxidizing atmosphere, such as carbon dioxide, nitrogen or other gas to the interior of the drying oven 14.V Member 34 may be provided with an inwardly extending shelf member 33 having openings 35 therein, if desired. Shelf member 33 will help to prevent liquid coating material 12 from being drawn into device 48, when it is a vacuum pump.

As is weli understood, by means of device 48, where device 48 is a vacuum pum-p, when the drying oven is being used the vacuum pump may be operated to substantially evacuate the interior of the drying oven 14 to thereby provide a substantially nonoxidizing atmosphere in the form of a vacuum. Of course, the drying oven 14 must be designed such that a vacuum may be drawn Without vdamage to the oven. However, it is also possible, 'as will be understood, to provide an atmosphere of either carbon dioxide, nitrogen, or other nonoxidizing gas within the drying oven 14 where the device 48 will be a storage tank of either carbon dioxide, nitrogen, or other gas which will 4be admitted under pressure into the interior of the drying oven 14. As will be understood, whichever is utilized, the interior of the drying ovenwill be substantially a nonoxidizing atmosphere for. providing curing of the coated wire 16 extending through the drying oven. Thus it will be apparent, that by means of a wire mill, similar to that disclosed in FIG. 1 of the drawing, it is possible to coat electrical conductors with an insulating liquid and to cure such insulating liquid in a nonoxidizing atmosphere to thereby substantially eliminate the volatile by-products of such coating without deteriorating or adversely attecting the physical properties of such coating.

Referring now toFIG. 2 of the drawing there is shown therein a curing oven substantially similar to that shown in FIG. 1, whereih hlm-insulated electrical conductors may be provided with a second cure, hereinafter called post-cure or post-cured, to substantially improve the electrical insulating properties of the film insulation without adversely alecting the physical properties of the iilm insulation. As is shown in FIG. 2, a drying oven -14 is provided having opposite refractory side walls 28', and electrical heating means 32 mounted in such refractory side walls, in the same manner as indicated with reference to FIG. 1. Similar end walls (not shown) are also provided. The top of the drying oven is provided with a top sealing member 5() having openings 42. therein, in the same manner as set forth with reference to FIG. 1. As will be understood, member 50 is sealed in an air-tight manner to the top of drying oven 14. The lower end of the drying oven is provided with a base member 52 substantially sealing the bottom of the drying oven and having openings 54 therein. Each 54 is provided with sealing means, in the form of rubber gaskets or the like 55.

In this apparatus, where it is desired to post-cure the previously cured film-insulated electrical conductors, each conductor 16' is mounted on a supply reel 18 and the conductor 16 is led from such supply reel through a guide sheave 20 throng-h openings 54 and rubber seals SS and into the drying oven 14. The insulated Wire 16' comes out of the drying oven through opening 42 in the ,cap member 40 around a guide sheave 24 and onto .a take-up reel 26. As will be understood, in this device it is only necessary that the wire 16' be passed through the drying over 14 once, inasmuch as there is no requirement in this drying oven to cure separate insulating coatings applied to the conductor, in the manner of the wire mill of FIG. 1. Further, as will be understood, the passage of the conductor through the drying oven 14' may be set at the particular desired time according to `the temperature of the oven and the type of electrical insulation -iilm on the Wire 16' to provide the desired cure `of such enamel film insulation, as will be more clearly specified hereinafter. Further, in a manner similar to that s'hown with reference to FIG. 1, an opening 44 is provided in the sealing member 50 of the drying oven, and the opening 44 has provided therein a neck member 46. Neck member 46 goes to :a device 48 which may either be a vacuum pump or a supply of carbon dioxide, nitrogen, or other gas as hereinbefore more fully discussed with reference to FIG. 1.

Sealing member 50 may also be provided with a second neck member 58 extending to an open chamber 60. As will be understood, when carbon dioxide, nitrogen 0r similar nonoxidizing gas is introduced into member 50 and oven 14 under pressure it is desirable to allow the excess gas and the volatile by-products of curing to be removed, preferably by bubbling through oil, glyccrine or the like, in the open chamber 60. Of course, neck 58 and chamber 68 will not be necessary, when 48 is a vacuum pump. In this manner, it will be understood that the insulated conductor 16' may be provided with a postcuring of the insulation thereon by means of drying oven 14' in a nonoxidizing atmosphere to thereby substantially improve the electrical characteristics of the insulating film.

FIGURE 2a illustrates a modified sealing means which' may be used with the curing ovens 14 or 14 of FIGURES l and 2, if desired. The modied sealing means of FIG- URE 2a is designed for use with a curing oven when the` oven is to be used in an evacuated condition. As shown, a sealing member 62 is provided, sealed at its lower end 64 to the drying oven 14. The upper portion of sealing member 62 denes a chamber 66 having upper and lower exits 68, 70, for passage of the conductor 16. Each of exits 68, 70 is sealed by rubber gaskets 72, 74, as shown. A neck member 76 is provided for introducing gas, such as carbon dioxide or nitrogen, into chamber 66 under pressure. A second neck member 78 is provided, leading to open chamber 80 to allow excess gas to be bubbled out through oil or glycerine.

Between exit 70 and sealed end 64 of member 62 is an elongated portion 82, which opens into the curing oven 14. An opening 84 is provided in portion 82 and a neck member 86 extends from such opening. A vacuum pump (not shown) may be secured to neck 86, thus providing for the evacuation of oven 14. It will be understood that the bottom of oven 14 may -be sealed in a manner similar to that shown in FIGURES 1 or 2. Alternatively, a sealing means similar to 62 may be provided, if desired. By means of the modified sealing member 62, a vacuum may be drawn within the oven 14, with the blanket of gas in chamber 66 preventing any outside `air being drawn through the rubber gaskets, such as 74, into the oven By use of this modified sealing means a better nonoxidizing atmosphere may be obtained in an evacuated drying oven for use with either the first cure or the post-cure of this invention.

It has been found that various types of insulating enamels which are used for coating electrical conductors may have their electrical properties substantially improved by means of the curing method of this invention. Some types of enamels whic-h have shown special promise in this field are enamels which are made from urea-formaldehydes, epoxies, the polyvinyl formals, and acrylics. Each of these types of enamel insulations has shown marked improvement in their electrical characteristics without deterioration of their physical characteristics when cured according to the method of this invention.

As is well known to those skilled in the electrical insulation art, one measure of the electrical properties of the insulation is the dissipation factor of such insulation. ln general, the dissipation factor provides a measure of the electrical resistance of the insulation. As a general rule, the electrical resistance of insulation decreases at increasing temperatures. An increase in the dissipation factor measures the decrease in electrical resistance With an increase in temperature, thus the lower the dissipation factor the better the electrical resistance of the insulation. To provide an indication of the electrical resistance of insulation it is usual to cite the dissipation factor of the insulation at the various temperatures.

FIGURE 3 shows curves which indicate the dissipation factor of various electrical conductors insulated with polyvinyl formal enamel which have been cured in the manner indicated below. The curve of FIG. 3 indicates the dissipation factor of each of these insulations over the various range of temperatures shown in the figure.

For example, considering curve A of FIG. 3 this curve represents polyvinyl formal coated wire which has lbeen cured in an air oven at approximately 250 degrees centigrade for minutes. The wire was provided with (8) coats of the polyvinyl formal enmel, with air curing of each coat. As can be seen by curve A, at 160 the dissipation factor is very low, approximately 7%, However. by the time the tempe-rature reaches approximately 190 C. the dissipation factor of this standard cured polyvinyl formal enamel is approximately 45%. Curve B is the curve of polyvinyl formal en amel coating of (8) coats in which each coat has been cured in a carbon dioxide atmosphere at a temperature of approximately 275 C. for 10 minutes for each coat. As can be seen, the dissipation factor at 160 is slightly lower than that for the standard cured polyvinyl formal enamel insulation, shown in curve A, and at 190 the dissipation factor is approximately 27% indicating an improvement over the regular curing of about 40%.

Curve C is an example of a vacuum cured .polyvinyl formal enamel coating which has been subjected to a postcure under a vacuum of approximately 8 mm. Hg at 275 C. for approximately 10 minutes. The wire of curve C was provided with (8) coats of polyvinyl formal enamel, with each coat being cured for approximately 10 minutes a-t 275 C. at a vacuum of 8 to 10 mm. of Hg. Whe-n tested at 180 C. this insulated wire had a dissipation factor of approximately 25%, showing only a small improvement over the coated wire of curve A. However, when subjected to post-curing, the electrical properties of this coated wire showed substantial improvement. As can be seen, when the .post-cure is provided, a substantial improvement is obtained in the dissipation factor such that .at approximately 200 C. the dissipation factor is only approximately 15% and at 220 the dissipation factor is less than 30%. Curve D of FIG. 3 is an exampleof the post-curing under a vacuum of 8 mm. Hg and 275 C. of the polyvinyl formal coated wire of curve B. As can be seen fro-m the curve D, the enamel insulation is very substantially improved by the addition of the post-cure to the previously cured polyvinyl formal under the CO2 blanket. ln this example the dissipation factor at 200 C. is less than 10%, while at 230 C. the dissipation factor is still under 30%. Thus as will be seen from FIG. 3, by means of curing of t-he `polyvinyl formal enamel in a nonoxidizing atmosphere, as disclosed in curve B, a substantial improvement may be obtained in its electrical insulating qualities. However, when an additional post-cure is provided even further substantial improvements are obtained in the electrical insulation properties of the polyvinyl formal -coated insulation.

Substantial work has been done in the area of post-euring of various types of enamel insulations which have previously been cured in the normal manner in an air oven. Various types of enamels have been tested in this fashion, and those made from polyesters, epoxies, polyvinyl formals, acrylics and urea-formaldehydes have shown substantial improvement in their electrical properties when subjected to the post-cure of this invention. The following are examples of the types of improvements which have been obtained:

Example I An electrical conductor coated with an acrylic enamel and cured in the standard fashion was subjected to a postcuring at 275 C. for approximatelylO mm. Hg. The dis-sipation factor of the insulation coating at C. dropped from greater than 50% to approximately 29%.

Example 1I Example III An epoxy wire enamel applied to an electrical conductor and cured in the normal Imanner was found to have a dissipation factor of 42% at 140 C. When this :sam-ple was treated at 325 C. for approximately 10- minutes at a vacu-um lof approximately 10 mm. Hg it was found to have a dissipation factor of 11% at 150 C. and 24% at 160 C. A second sample of this wire was postcured at 350 C. -for approximately 1.0 minutes in a vacnum of approximately 10 mm. Hg. This second sample was found to have dissipation factor yof 11% at 160 C. and 45% at 180 C.

Example IV A wire coated with polyvinyl formal enamel in the usual manner, cured in the usual way was found to have a dissipation factor of 12% at 170 C. and approximately 45% at 190 C. (See curve A, FIGURE 3).' One portion -of this wire was post-cured at 275 C. for approximately 10 lminutes under a vacuum of 10 Imm. Hg. This 7 post-cured wire was vfound to have a dissipation fact-or of 7% at 170 C., 22% at 190 C. and 39% at 200 A second sample was post-cured at 350 C. for approximately 10 minutes lunder a vacuum of 10 mm. Hg and was 'found to have a dissipation factor of only 4% at 170 C., 10% at 190 C. and 33% at 220 C.

Example V A wire conductor was coated with a urea-formaldehyde-phenolic enamel according to standard practice had a dissipation factor of 33% at 160 C. and a 50% dissipation factor at 168 C. This sample was then postcured at 270 C. for approximately 10 minutes at a vacuum of 10 mm. Hg. After .post-curing its dissipation factor was only 3% at 160 C., 12% at 180 C. and did not reach 50% dissipation fact-or until the temperature was increased to 206 C. 'All ofthe above wire samples were still flexible after the post-curing, as is indicated -by a one-time diameter wrap after the post-curing. Thus .from the above it is seen that post-curing of various types of electrical insulating enamel will .provi-de a substantial improve-ment in their electrical properties without any detrimental elects t-o their physical properties.

Referring now to FIG. 4 of the drawings, there is shown curves Aindicating, the post-cure treatment of polyvinyl formal coated wire according to this invention. Curve A of FIG. 4 is polyvinyl formal coated wire which has been cured in the usual manner. As can be seen, the curve A of FIG. 4 is essentially equivalent to curve A of FIG. 3. Curve B of FIG. 4 is an example of polyvinyl formal coated wire, cured as in curve A, which has -been post-cured under a C02 atmosphere for approximately 10 minutes at 275 C. As can be seen, the dissipation lfactor of this insulation is substantially improved over that of curve A. Curve C is another sample of the enamel wire when post-cured at 275 C. for approximately 10 minutes under an N2 atmosphere. Again, it can be seen that a substantial improvement is obtained in the electrical properties of the insulation when subjected to the post-cure of this invention. Curves D and E are examples of polyvinyl formal coated wire which have been postcured at approximately 350 C. rfor 1.0 minutes, curve D -under a vacuum of S mm. Hg and curve E funder a vacuum of 250 microns of Hg. As can be seen, the improvement in the dissipation factor of these enamels is essentially similar under these vacuum treatment post-cures.

FIGURE 5 shows the curve of a post-cure treatment of an epoxy enamel on an electrical conductor which has been subject to a vacuum curing of approximately 8 mm. Hg for minutes at 275 C. Curve A is the epoxy enamel insulation subject to the normal curing, while curve B is the ep-oxy enamel insulation subjected to the post-cure of this invention. As can be seen, the epoxy enamel when post-cured according to this invention shows a marked improvement of its electrical properties over the standard cure ot curve A.

FIGURE 6 shows the similar results obtained for polyster enamel on an electrical conductor when subjected to a vacuum post-curing of approxi-mately 10 mm. Hg at 275 C. for approximately 10 minutes. As curve A is the polyester enamel with the normal cure, and curve B is the polyester enamel subjected to the post-cure of this invention, again, it is readily apparent the substantial improvement in electrical properties which can `be obtained by means of the post-curing of this invention.

In order to further show the substantial improvements which are obtained by the post-curing of this invention, the curves of FIG. 7 have been provided. Curve A of FIG. 7 shows the dissipation -factor of polyvinyl formal coated electrical conductors, cured in the usual manner, which have been aged at 175 C. for the days indicated. Curve B is the dissipation factor of polyvinyl formal enamel-insulated electrical conductors which have been post-cured at a vacuum of approximately 8 mm. Hg at 275 C. for 10 minutes. These also are aged at 175 C. for the days indicated. The dissipation factor of the different coils has been determined at 175 C. at the end Vof the various days. As can :be seen, the dissipation factor of the post-cured polyvinyl lformal coated coil is substantially lower than that of the normally cured polyvinyl formal enamel and it retains its low dissipation factor for approximately days compa-red to the approximate 20 days of the standard cured polyvinyl formal enamel coated coils. From this it can be seen Ithat the aging characteristics of the insulating enamel is also substantially improved by means of the post-cure of this invention.

In the various examples set forth above, the various times and temperatures, as well as the nonoxidizing atmosphere have been set forth for various types of enamel coated electrical conductors. From the various information provided it will Vbe apparent that the maximum temperature for .post-curing of enamel coated wire will ybe the temperature at which the particular enamel will decompose. The lower temperatures at which proper postcuring may be obtained will depend greatly upon the ty-pe of enamel, the limit of the oven, and the time of travel of the Wire through the oven. Obviously, the temperatures must be suciently high to obtain a post-cure of the enamel on a dynamic system. Thus the enamel coated wire must run through the oven and the temperatures must -be suflciently high over the period of travel to provide the desired post-cure. For example, as above indicated with polyvinyl formal enamel it is necessary to provide at least 10 minutes of post-cure at 250 C. regardless of the type of nonoxidizing atmosphere. However, where the temperature is raised up to 350 C. -for l0 minutes substantially better results are obtained, as has been shown. In a similar fashion, the higher ternperatures will in general provide better results, that is, a substantially better increase in the electrical properties of the insulation, as long as the higher temperature does not exceed the temperature at which the particular type of enamel will decompose. In general, it has been found that approximately 10 minutes at this high temperature is su'cient to provi-de a post-cure which will .provide an excellent increase in the electrical properties of the various types of enamel. Of course, with the higher temperatures, substantially improved results may :be obtained where the post-cure is less than ten minutes. However, as will be apparent, depending upon the desired propertles lower temperatures may be lutilized and obviously longer times may Ibe provided in the curing oven to obtain the desired increase in electrical properties. Therefore, while there fhas Ibeen shown and described various preferred embodiments of this invention, and while specific times, temperatures, and types of nonoxidizing atmospheres have been set forth, it will be apparent to those skilled in the art that Ivarious changes may :be made in the times, temperatures, and atmospheres without departing -from the spirit and scope of this invention. The 1nventi0n to .be protected is defined in the following claims.

What is claimed as new and which it is desired to Secure by Letters Patent of the United States is:

1. A method of curing a film insulation selected from the group consisting of epoxies, polyesters, acrylics, polylvinyl -formals and urea-formaldehydes, in which the lm is coated on a conductor and the conductor is heated in a nonoxidizing atmosphere at a temperature below the decomposing temperature of said film insulation 'said temperature being at least 250 C. for approximately l0 minutes.

2. A method of improving the film insulation on an electrical conductor, where the conductor has a lm insulation selected Afrom the lgroup consisting of epoxies, polyesters, acrylics, polyvinyl formals and urea-formaldehydes, comprising the steps of subjecting the insulated conductor to a post-cure in a nonoxidizing atmosphere at a temperature less than the decomposing temperature of said lm insulation said temperature being at least 250 C. for approximately 10 minutes.

- 3. A method of providing an improved polyvinyl -formal film insulation on an electr-ical conductor comprising the steps -of passing the conductor through a bath of 5 liquid polyvinyl formal insulating material, then passing the coated conductor through a curing Voven wherein the curing oven is lprovided with a non-oxidizing atmosphere and in which the curing oven is maintained at a temperature not less than 250 C. `and not in excess of 350 C. and each segment of the coated conductor passes through the oven in not more than 10 minutes.

4. A method of providing an improved polyvinyl -formal film insulation on an electrical conductor comprising the steps of passing the conduct-or through a bath of liquid polyvinyl formal insulating material then .passing the coated conductor through a curing oven wherein the curing oven is provided with a nonoxidizing atmosphere and then post curing the ins-ulated conductor in a nonoxidizing atmosphere at a temperature not less than 250 C. and not in excess of the decomposing temperature of the polyvinyl formal insulation for approximately 10 minutes.

References Cited bythe Examiner UNITED STATES PATENTS 2,970,936 2/ 1961 Richardson 117-218 X FOREIGN PATENTS 16,338 3/1956 Germany. 730,289 5/ 1955 Great Britain.

15 ALFRED L. LEAVI'IT, Primary Examiner.

WILLIAM L. JARVIS, Examiner. 

1. A METHOD OF CURING A FILM INSULATION ELECTED FROM THE GROUP CONSISTING OF EPOXIES, POLYESTERS, ACRYLICS, POLYVINYL FORMALS AND UREA-FORMADEHYDES, IN WHICH THE FILM IS COATED ON A CONDUCTOR AND THE CONDUCTOR IS HEATED IN A NONOXIDIZING ATMOSPHERE AT A TEMPERATURE BELOW THE DECOMPOSITING TEMPERATURE OF SAID FILM INSULATION SAID TEMPERATURE BEING AT LEAST 250*C. FOR APPROXIMATELY 10 MINUTES. 